Transducer with impedance-matching bridge



Oct. 16, 1962 R. ADLER 3,053,539

TRANSDUCER WITH IMPEDANCE-MATCHING BRIDGE Filed May 15, 195a 3Sheets-Sheet l /NVE/Vr0/? Robe r6 (/Zdler ATTORf/EK Oct. 16, 1962 R.ADLER 3,058,539

TRANSDUCER WITH IMPEDANCE-MATCHING BRIDGE Filed May 15, 1958 3Sheets-Sheet 2 llVl/E/VTUR Roberi (/ZciZer ATTORNEY United States PatentOfiice 3,058,539 Patented Oct. 16, 1962 3,058,539 TRANSDUQER WITHlMPEDANCE-MATCIHN G BRIDGE Robert Adler, Nortlifield, Ill., assignor toZenith Radio Corporation, a corporation of Delaware Filed May 15, 1958,Ser. No. 735,548 18 Claims. (Cl. 18l.5)

This invention relates to electromechanical transducers in general andis particularly concerned with the construction of an improvedtransducer arrangement for effecting energy transfer to or from awave-propagating medium which exhibits a materially different impedanceat a given signal frequency than the mechanical impedance of thetransducer itself.

Transducers of the type under consideration are frequently employed forthe purpose of transferring signal energy from air or otherwave-propagating medium to a wave signal receiver, but because of thereciprocal prop erties of wave signal reception and transmission, theteachings hereof may be advantageously applied to transmitting as wellas to receiving systems. The invention is especially suited for use inremote control system-s wherein a controlled or satellite stationexecutes any of several controlled functions depending upon thefrequency of command signals transmitted by a controlling station and,for convenience of illustration, the ensuing detailed description willbe directed to a microphone and transmitter for that application.

A very successful remote control system featuring the use of amultiplicity of command signals, distinguished from one another by thefrequency of the transmitted energy, is the subject of US. LettersPatent 2,817,025, issued on December 17, 1957 in the name of RobertAdler and assigned to the same assignee as the present invention. Asthere described, the controlled device is a television receiver in whichthe functions of on-oif switching, channel selection and sound mutingare accomplished through remote control. Channel selection is achievedby appropriate energization of a bi-directional motor which drives aturret-type tuner in either clockwise or counterclockwise direction.That arrangement requires four different command signals, two to permitcontrol of the tuning motor in both directions and one each for theon-oif and sound muting. These commands take the form of bursts orpulses of acoustical energy transmitted upon the actuation of ultrasonicradiators contained within a compact, lightweight and portabletransmitting unit. Each sonic radiator is a longitudinal-mode passivevibrator, for example, an aluminum rod supported at a centrally locatednodal point and a striker for impinging against the free end of the rod.Since four signals are required, the transmitter includes four such rodsand the length of each, in combination with the velocity of signalpropagation therein, determines the signal frequency.

It is necessary that the controlled receiver he as insensitive to falseactuation as practicable or, expressed in other words, it must berelatively free from actuation in response to interference and spurioussignals that may be encountered in the vicinity of the controlledreceiver. To that end, the controlled station has a narrow acceptanceband and is thus able to reject, on the basis of frequencydiscrimination, interfering signals which do not fall within itsacceptance band. By way of illustration, the command signals may beincluded in the frequency range from 38 to 42 kilocycles and thecontrolled device only responds to this small band of frequencies.

In the Adler patent, further frequency selectivity within thisrestricted band is achieved by the use of frequencydiscriminatorcircuits to which command signals are applied by a sonic receiver,specifically, a microphone of the electrostatic type. An electrostaticmicrophone of improved construction particularly valuable for use as asonic receiver in such a system is described and claimed in a copendingapplication of Robert Adler, Serial No. 661,348 filed May 24, 1957 nowPatent No. 2,908,772 and likewise assigned to the same assignee as thepresent invention. It is distinguished from commercially availableelectrostatic microphones by its high sensitivity in the above-mentionedfrequency range in the vicinity of 40 kilocycles and it has a bandwidthof 8 kilocycles; however, it would be desirable to employ apiezoelectric microphone as the sonic receiver because of itspotentially greater sensitivity, because its bandwidth may beconveniently confined to narrow limits and because it lends itself moreconveniently to utilization with transistor amplifiers to which thesonic receiver may be coupled.

Prior attempts to adapt piezoelectric microphones to this applicationhave not attained adequate efficiency nor have they realized thepotential of that type microphone, primarily because of the difficultyof effecting a satisfactory impedance-matching relation between air andthe piezoelectric device, which has a very high mechanical impedancerelative to that of air. By mechanical impedance is meant the quotientof force divided by velocity in a vibratory system, in complete analogywith the definition of electrical impedance as the quotient of voltagedivided by current in an electrical circuit. It has been proposed, forexample, that a piezoelectric element made from a material such asbarium titanate be polarized transversely in two adjacent semi-circularportions which are connected in series. Such a microphone, if usedwithout a tuning inductance to tune out the capacitance which isrepresents, has a sharp frequency response, exhibiting high sensitivityat the frequency of mechanical resonance but having a rapidly decreasingsensitivity at all other frequencies. If the capacitance of themicrophone is tuned out by means of a tuning inductance, a responsecharacteristic results which is of the double-peaked or saddle-shapedvariety associated with a coupled pair of tuned circuits having thesamefrequency of resonance. That characteristic may be shaped and madefairly flat through the expedient of electrical damping. This approachto the microphone problem has not proved out well; the inductancerequired for tuning is very large and costly and must be carefullyshielded, which adds further to the cost. The sensitivity of this deviceis not sufiiciently higher than that realized with an electrostaticmicrophone of the type described in the afore-identified application towarrant the increased expense.

Accordingly, it is an object of the present invention to provide anelectromechanical transducer, such as apiezoelectric microphone, whichavoids the disadvantages of prior structures and has enhancedsensitivity.

It is another object of the invention to provide an improvedelectromechanical transducer in which an auxiliary mechanical resonator,having an impedance intermediate that of the principal transducingdevice and the medium in respect of which signal energy is to betransferred, greatly enhances the signal-transferring properties of thearrangement.

It is a particular object of the invention to provide a piezoelectricmicrophone of greatly improved efiiciency attained through improvedmechanical impedance matching between the microphone and the medium,such as air, to which it is coupled.

It is a specific object of the invention to provide an improvedstructure for efficiently coupling or matching an electromechanicaltransducer to a wave-propagating memechanical impedance of thetransducer itself.

Another specific object of the invention is to provide a microphone,including a piezoelectric element, suitable for convenient utilizationwith an amplifying stage employing either vacuum tube or transistoramplifying devices.

Still another specific object of the invention is to provide apiezoelectric ceramic microphone having greatly improved sensitivitythroughout a well-defined pass band.

An electromechanical transducer arrangement, constructed in accordancewith the invention, is especially suited for effecting transfer ofenergy with respect to a medium, such as air, which ha a predeterminedimpedance at a given signal frequency. The arrangement comprises atransducer element having a frequency of mechanical resonanceapproximately equal to the signal frequency and having a mechanicalimpedance at that frequency which is high relative to the impedance ofthe aforesaid medium. The arrangement further includes a resonant,mechanical impedance-transformation device supported for vibration in aflexural mode for coupling the transducer to the medium. The mechanicaldevice has an impedance at the signal frequency intermediate that of thetransducer and the medium; it has a frequency of mechanical resonancecorresponding to that of the transducer, and it is mechanicallyconnected thereto to constitute therewith a mechanical impedancetransformer which is the analogue of an electrical impedance transformerfor converting between parallel and series resonant impedance relationsin a resonant circuit.

The teachings of the invention have general application to transducerarrangements of the type in which the impedance of the transducerelement is high in comparison with the impedance of air or other mediumwith respect to which a transfer of signal energy is desired.Accordingly, the inventive concept may be advantageously employed wherethe transducer is an electromagnetic or electrodynamic device, apiezoelectric element, a longitudinal-mode vibrator such as an aluminumrod or even a magnetostrictive device. All such transducers as normallyconstructed are too heavy, that is to say represent too high animpedance, to be matched etficiently to air except for particularfrequency ranges which generally vary with the type transducer. Thedifficulty of bridging the dissimilar impedances of such devices and airis obviated through the present invention which contemplates the use ofan auxiliary mechanical resonator as an impedance-matching devicefunctioning in a manner analogous to an electrical resonant circuitwhich acts as a transformer converting between parallel and seriesresonant impedance relations. material to follow, detailed considerationis given to the application of this principle to a piezoelectricmicrophone and to a sonic transmitter of the magetostrictive type.

In both these applications, it is convenient and effective to employ abridge element as the auxiliary resonator. The bridge is formed of athin strip of metal, such as aluminum, proportioned to have the samefrequency of mechanical resonance as the transducing devic which it isto couple to air. While this is a particularly satisfactory form ofresonator to employ, a variety of other shapes of resonator devices isavailable and certain of them are also discussed in the detaileddescription and illustrated in the drawings.

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. The organizationand manner of operation of the invention, together with further objectsand advantages thereof, may best be understood by reference to thefollowing description taken in connection with the accompanyingdrawings, in the several figures of which like reference numeralsidentify like elements, and in which:

FIGURE 1 is a view, in vertical section, of a microphone assemblyembodying the present invention;

In the descriptive FIGURES 2 and 3 are additional views taken asindicated by section lines 2-2 and 33 in FIGURE 1;

FIGURE 4 is an enlarged sectional view of the piezoelectric elementtaken as indicated by section line 44 in FIGURE 3;

FIGURE 5 is an exploded perspective view revealing component details ofthe microphone assembly;

FIGURE 6 is an enlarged fragmentary cross-sectional view of the centralregion of FIGURE 1;

FIGURE 7 is a view similar to FIGURE 6 but with a modification of one ofthe parts;

FIGURE 8 is a schematic circuit diagram of the electrical analogue ofthe microphone assembly;

FIGURE 9 illustrates the frequency response characteristic of themicrophone;

FIGURE 10 represents a modification of the microphon structure;

FIGURES lla-b, 12a-b and 13a-b are enlarged views of different shapes ofauxiliary resonators that may be used for impedance matching; and

FIGURE 14 is a schematic representation of a magnetostrictive transducerembodying the invention.

Referring now more particularly to the drawings, especially FIGURE 1,the electromechanical transducing arrangement there represented is adevice for effecting the transfer of energy with respect to a mediumwhich propagates a signal wave of a given frequency and which exhibits aknown impedance at that frequency. As indicated above, detailedconsideration is to be given to a microphone for receiving ultrasonicenergy radiated from a transmitter and propagated to the receiverthrough air. Air, as any wave signal propagating medium, has acharacteristic impedance at sonic frequencies which may be defined asthe quotient of the pressure by the particle velocity in the sound wave.Stated more fully, characteristic impedance is the ratio of force tovelocity of volume flow in a distributed medium capable of propagatingsound, particularly in air. By referring both force and velocity to acommon unit of cross-sectional area, characteristic impedance may alsobe defined as the ratio of sound pressure to the amount of volumeflowing through one unit of area per unit time. Since this latter amountis equal to the particle velocity, characteritic impedance is also equalto the quotient of the pressure divided by the particle velocity, asstated initially. The great diificulty encountered in achieving apiezoelectric ceramic microphone of high sensitivity is in effecting agood impedance match between the piezoelectric element and the air inspite of the fact that the mechanical impedance of the ceramic elementis very much higher than the impedance of air at the signal frequency.Air is a very light medium, is easy to move and experiences a largedisplacement in response to a relatively small moving force, whereas apiezoelectric transducer is very hard to move and is a heavy devicecompared with air. It yields but little and experiences only a smalldisplacement in response to the application of a relatively high movingforce.

A salient feature of the invention is a structure which, in effect,bridges such dissimilar impedances to attain an efiicient couplingtherebetween. This is accomplished through the agency of an auxiliarymechanical device which is itself heavier than air and yet lighter thanthe piezoelectric element. The matching structure is furthercharacterized by a frequency of mechanical resonance matching that ofthe piezoelectric element and it is connected to that element to definewith it a mechanical structure which is the full equivalent of anelectrical impedance transformation network converting between seriesand parallel impedance in a resonant circuit.

More specifically, the microphone structure of the embodiment of theinvention under consideration comprises a dimorphic piezoelectricelement 10.

A dimorphic piezoelectric device is one which has at least twopiezoelectric wafers or components having such relative polarization andelectrical coupling through their electrode structures that theelectrical signals developed, in response to bending, are additivelyrelated. As detailed in FIGURES 4 and 6, the dimorph is made up of twowafers 11, 12 of a material having a high electromechanical couplingfactor in order to provide a high efficiency of energy transfer betweenthe electrical and mechanical actions. The class of materials known astitanates is particularly suited for use in the wafers. Suchcompositions as barium titanate and barium strontium titanate arefrequently employed. The wafers are rectangular in shape and havesilvered surfaces '13, 13 extending along the upper and lower margins inthe width dimension, these surfaces also preferably having aninterconnecting portion extending in the thickness direction. The silvermay be applied through a silkscreen process and fired in situs to bondit to the wafer. The silvered surfaces are the electrodes through whicha unidirectional polarizing potential is applied to the wafer for thepurpose of establishing a permanent polarization in a given direction,in the longitudinal mode for the structure shown in the drawing.

After the wafer pair have been polarized, they are assembled in therelative orientation shown in FIGURES 4 and 6, namely, with opposeddirections of polarization indicated by the full-line arrows in FIGURE4. Thin flexible electrode leads l4 and 15 are interposed at theapproximate locations of the nodal axes assuming the dimorph to besupported for vibration in the lowest flexural mode. The locations ofthe nodes are designated by the broken construction lines NN; they arespaced approximately 22 /2% of the full length of the wafers from theedges of the wafers. The leads are formed of coined silver and arebonded to the contiguous silvered surfaces through a conductive cementin a pressing operation which integrates the sandwich or dimorphassembly. It is not essential that there be conductive connectionsbetween leads 14, 15 and the adjacent silvered surfaces, but bestefiiciency results if such connections are established.

Leads 14, 15 serve to support the dimorph from a mounting structure tobe described presently and have the advantage of relieving therequirement that the supports be precisely located at the nodal points.The dimorph has its least displacement or movement at the nodal pointsso that if it is supported at such points the support contributes theleast amount of detuning and damping. Thin flexible support leadsaccommodate the bending forces encountered if the supports are not, infact, precisely located with respect to the flexural nodes.

The active portion of the dimorph is that included be tween the silveredsurfaces or between broken construction lines NN and it is not necessaryto polarize the remaining edge portions of the wafers. The assembleddimorph is dimensioned to have a frequency of fundamental flexuralresonance equal to a reference signal frequency and adjustment as tofrequency of mechanical resonance is achieved by grinding. In someconstructions, opposed external broad surfaces are subjected to grindingin order to arrive at the desired frequency which, of course, removesthe silvered coating 13. A more expedient adjustment as to frequency isobtained by edge grinding of the dimorph. This, too, tends to remove aportion of the silvering but only that which is on the edge of thewafers and this is not significant or important.

A mechanical impedance transformation device is employed for couplingthe dimorph to air, that is to say, for effecting an efficient impedancematch between the dimorph and air. It is an auxiliary resonator havingan impedance at the signal frequency between that of dimorph and air andmay be constructed in any of a variety of shapes. In a preferredembodiment, the resonator is a bridge member formed of a thin strip ofmetal, such as aluminum, of U-shape shown in detail in FIGURES 4 and 6.It has a bight portion 20 and legs 21, 21 normal to portion 20. Thelegs, which are secured to the surface G of the dimorph as presently tobe described, support portion 20 at each end permitting it to vibrate ina flexural mode and section 20 is proportioned to have a frequency ofmechanical resonance coresponding to the frequency of mechanicalresonance of the dimorph.

The bridge is mechanically connected to the dimorph in such a manner asto constitute therewith a mechanical impedance transformer which is ananalogue of an electrical resonant circuit transforming between paralleland series impedance relations in a resonant circuit, as mentionedhereinabove. The mechanical connection is effected through legs 21, 21.If desired, terminal portions 21a of the legs may be bent to constitutesupporting shoes for the bridge as indicated in FIGURE 7. Structures ofthis type have been employed with the connection to the dimorphestablished by cementing the shoes to the upper surface. It is found,however, that there is a tendency toward instability if the assemblingprocess is not carefully carried out because cement may seep between theshoe and the dimorph. To avoid this possibility, the free ends of thelegs are cemented to the dimorph in the manner of FIGURE 6.

It is preferred to orient the dimorph and the bridge to arrive at amechanical structure which provides a high ratio of impedancetransformation. Specifically, the loop or anti-node of the dimorph isparallel to the construction lines NN of FIGURE 4 and the orientation ofthe bridge is chosen so that its loop or anti-node is normal orperpendicular to that of the dimorph. With reference to FIGURE 4, theanti-node of the bridge is in a plane parallel to the plane of thedrawing. The anti-node of the dimorph is centered between nodal pointsNN and therefore the lowest mechanical impedance is encountered if thebending force is applied thereto. Accordingly, the legs 21 of the bridgeare affixed to the dimorph in this region. In theory, legs 21 of thebridge should be tapered to provide point contacts at the anti-nodepoint but this would result in a mechanically-Weak structure. On theother hand, a line contact between the legs of the bridge and thedimorph would tend to stiffen the di morph in this region which isnormally subject to maximum bending and it is preferred to avoidunnecessary stiffening. Consequently, legs 21 are tapered in the mannerrepresented in FIGURE 4 to achieve a mechanicallystrong union betweenthe bridge and the dimorph consist out with minimum stiffening of thedimorph resulting from the mechanical junction. Preferably, the bridgestructure is centered with respect to the anti-node, that is, withrespect to the central region of maximum bending or displacement of thedimorph.

The dimorph and bridge assembly is housed within a mounting structurecomprising a base 25 and a cap 26, both formed of an insulating materialsuch as polystyrene. While the assembled mount appears in cross-sectionwith FIGURE 1, the details of its principal components are more clearlyrevealed in FIGURE 5 wherein it appears that base 25 has acentrally-located recessed area 27. It also has a pair of parallel,longitudinally extending slots 28, 28 and 29, 29 leading from oppositesides of recess 2'7. .These slots are dimensioned to receive theelectrode leads.

The detail of FIGURE 5 shows that leads 14, 15 collectively define agenerally rectangular frame to which the dimorph-bridge assembly iscentrally affixed. Each side of this frame is cut as indicated at points343, 31 so that the electrodes are not conductively connected to oneanother after the structure is assembled. Actually, a complete andcontinuous framework is provided by elements 14, 15 during thefabrication of the dimorph to facilitate precise assembly, but as soonas this is finished the frame is broken at points 3%, 31.

Cap 26 has a central recess 35 which, in conjunction with recess 27 ofbase 25, defines a cavity for receiving the dimorph-bridge assembly withthe bridge positioned within a close-fitting window 36 cut out of thecenter portion of recess 35. A pair of ribs 37, 37 and 38, 38 projectfrom the inner surface of cap 26 to be received within slots 28, 28 andZ9, 29 of the base to complete a clamping engagement of electrode leads14, 15. In this fashion, the dimorph and bridge assembly is mechanicallysupported for vibration in a fiexural mode. Dowel pins 39, 39 projectfrom the inner face of base 25 and are received in apertures 49, 49 ofthe cap to facilitate assembling the housing structure. A pair of screws41, 41 feed through aligned apertures in both parts 25, 26 of themounting structure and nuts 42, 42 drawn up on the screws lock thestructure into an integrated assembly.

The bight portion 20 of the bridge has a smaller surface area than theparallel surface of the dimorph. More significantly, it is smallcompared with the wavelength in air at the signal frequency and iscoupled to air through a tapered horn 45 shown in FIGURES 1, 2 and 5.The dimensions of the mouth of the horn are selected in accordance withthe requirements of the signalling system in which the microphone is tobe employed. It has been assumed that the microphone under considerationis intended for use in a remote control system of the type described inaforesaid Patent No. 2,817,025 for controlling certain operatingconditions of a television receiver. The larger the mouth dimensions ofthe horn, the more signal energy it intercepts but, at the same time,the more directional it becomes. In the environment in which themicrophone is to be employed, only a small directivity can be toleratedin a horizontal direction because the remote control transmitter shouldbe effective over as wide an angle with respect to the mouth of the hornas possible. Much greater directivity can be accommodated in a verticaldirection because there will be only a few instances in which thetransmitter will have any considerable elevation with respect to themicrophone. An acceptable compromise of directivity with respect to thequantity of energy intercepted is achieved by a dimension H of onewavelength in the horizontal direction and a dimension V of twowavelengths in the vertical direction, as represented in FIGURE 2. Thethroat dimensions of the horn are selected to fit closely about bridge20. The length is chosen with relation to a limiting angle at whichincident sound would be reflected out of the horn. The steepness of thehorn is made less than the angle at which any such reflection isencountered. The horn is shown with a straight taper, but other types oftaper such as an exponential taper may be used.

The throat of the horn communicates with bight 20 of the bridge throughan acoustical impedance-matching sec tion 46 which is a tubular spacehaving a length of onequarter wavelength at the signal frequency. FIGURE6 includes an enlarged showing of quarter-wave section 46 which is seento have a terminal portion projecting into window 36 of cap 26. The hornmay also be formed of an insulating material such as polystyrene and itsmechanical elements, aside from those which collectively define the hornand quarter-wave terminating section, are provided for the purpose ofincreasing the mechanical strength of the structure. A flange plate 50mechanically secured to the mouth of the horn bears a cushion 51 ofsponge rubber material serving as a bumper to protect the structure whenin position on a control chassis with the mouth of the horn injuxtaposed relation to a sound opening through which sonic commandsignals make their way to the microphone.

The horn and flange assembly is integrated with the dimorph housing by apair of machine screws 52, 52 extending through a pedestal 53 formedintegrally with the horn. The mounting screws extend through holes 54,54 located in flange projections of cap 26 and thread into tapped holesprovided in a metallic supporting plate 55. Dowel pins 48, 48 extendingfrom base plate 53 are received by apertures 49, 49 in cap 26, andfacilitate assembling the horn to mounting structure 25, 26. Plate 55constitutes an electrical ground plane and electrode lead 15 of thedimorph is grounded thereto by a machine screw 56. The other electrodelead 14 threads over machine screw 41 serving as a terminal post towhich an output lead 57 connects, as illustrated in FIGURE 1. Obviouslythe pictorial view of FIGURE 5 has been selected to reveal the detailsof the structure and should not be taken to mean a curvilinearstructure. Reference to the assembly view of FIGURE 1 makes clear thefact that this is a linear structure.

The entire assembly is enclosed within shield receptacle 6% mountingplate being mechanically secured thereto by means of tabs 61 projectingfrom the plate and extending through slots in the casing. The tabs maybe soldered to the casing or bent over to lock the confined assembly inposition. A machine screw and nut assembly 63 extending through an endplate of receptacle and receiver chassis 64 facilitates mounting thestructure in operating position.

In the operation of the described microphone, an ultrasonic radiatedsignal reaching horn 45 is channeled through the quarter-waveimpedance-matching section 46 to impinge upon bight portion 20 of themetal bridge.

The received sonic signal constitutes a small force ap-' plied to theresonant section of the bridge to set it into fiexural-mode vibration.The small force applied at the center of the bridge produces adisplacement there of large amplitude and results in a much larger forceappearing at the supports of the resonant section, namely, at legs 21,21. The large force developed at support points 21, 21 is applied to thecenter of the dimorph and results in displacement or bending of thedimorph. This dis placement is of much smaller amplitude than thatexperienced by the bridge in View of the high mechanical impedance ofthe dimorph but, nonetheless, establishes fiexura l-mode vibrationstherein. The mechanical bending action, through piezoelectricconversion, results in electrical signals appearing on electrode leads14, 15. While lead 15 is maintained at ground potential, the variationsin potential of lead 14 represent the received signal and may be appliedover conductor 57 to an amplifier.

The described mechanical impedance transformation from a condition oflarge amplitude displacement and low driving force acting upon bridge 20to a condition of low amplitude displacement and high driving forceacting upon the dimorph is analogous to the electrical transformationachieved with the electrical analogue represented in FIGURE 8. Theelectrical analogues of the resonant bridge and dimorph are indicated inbrokenline boxes with the legends Bridge and BaTiO Dimorphy 10 has asits electrical equivalent the series combination of a capacitor 66a andan inductor 66b, and bridge 2t) has as its electrical equivalent theparallel combination of a capacitor 67a and an inductor 67b. The entireequivalent network takes the form of the parallel combination ofcapacitor 67a and inductor 6717 with the input terminals being in serieswith inductor 67b, that parallel combination being in turn paralleled bythe series combination of capacitor 66a, inductor 66b and the outputterminals. The indicated high current and low voltage at the inputterminals with the reverse at the output terminals are the analogue ofthe mechanical relationships present in the apparatus herein disclosed,current corresponding to velocity and voltage to force.

Since the dimorph and the bridge have the same frequency of mechanicalresonance, they establish for the transducer a frequency response curve68 of the type represented in FIGURE 9. It is peaked at two frequenciesequally spaced from the signal frequency i and is analogous to thedouble-peaked or saddle-shaped characteristic of two coupled electnicalcircuits resonant at the same signal frequency. Were the microphoneoperated in air at reduced pressure, the response characteristic in theregion between the peak responses would be generally as indicated inbroken-construction line 69. Specifically, the response would consist oftwo very sharp peaks with little, if any, amplitude therebetween. Thatreduction in response would be occasioned by the fact that the structurehas no inherent damping. However, the response may be shaped by loadingeither or both of the resonant bodies. It is possible, by means ofmechanical and/or electrical loading, to, in effect, fill in the saddleor shape the characteristic to achieve a substantially flat responsethroughout the bandwidth of the microphone. More particularly, dampingmay be achieved by mechanically loading the bridge or by elec tricallyloading the dimorph. In the arrangement specifically illustrated, airresistance eifects mechanical loading of the bridge to attain a morenearly uniform response over the acceptance band of the microphone.

By Way of summary, bridge 20 performs two important functions whichcontribute to the outstanding properties of the microphone: (1) itserves as an impedance transformer between the dimorph and air whichhave materially different mechanical impedances or, expressed in otherwords, it is a mechanism for efficiently coupling the dimorph to air;and (2) in conjunction with the mechanical resonant properties of thedimorph, it establishes a response characteristic which is the analogueof two coupled tuned circuits having a common resonance frequency. Theband-pass of the device may be controlled by adjustment of the ratio ofmass to stiffness of the two mechanically resonant bodies.

In one embodiment of the microphone constructed and successfullyoperated, the dimorph and bridge had the following dimensions andproperties:

onance 4O kilocycles. Bridge 20 4 mils aluminum.

Bight portion .100 by .119 inch Legs 70 mils high, width tapered from100 to 70 mils. Frequency of mechanical resonance 40 kilocycles.Frequency separation of peaks of response curve 5.3lr1locycles.Sensitivity between peaks 4'4 to 45 decibels be low '1 volt per dyne/cm.

The microphone rep 'resented a capacitance of approximately 16micromicrofarads and was employed to drive a pentode amplifier for whichthe input electrode capacitance and associated stray capacitance wasapproximately micromicrofarads.

As suggested above, the dimorph may be loaded electrically to enhancethe mechanical damping produced by the air. Where electrical loading isto be employed, the capacitance represented by the microphone is firsttuned out by a tuning inductance or coil and then any suitable amount ofresistance damping applied. Damping of this type is not necessary, andis not resorted to when coupling the microphone to a vacuum tubeamplifier but it is employed when working the microphone into atransistor amplifier in view of the low input impedance of this type ofamplifier. In arranging the microphone to drive a transistor amplifier,it is preferred to use transverse rather than longitudinal-modepolarization of the dimorph because this increases the capacitancerepresented by the microphone, reduces the amount of tuning inductancerequired and makes it easier to attain a suitable measure of electricaldamping from the input resistance of the amplifier.

Another mode of polarization which may be used in certain constructionsis the so-called reverse alternate mode described and claimed in patentRe. 23,813, reissued on April 20, 1954 in the name of Robert Adler andassigned to the same assignee as the present invention. Where that typeof polanization is to be used, wafer 11 could, for example, beconsidered to have two sections, one being the portion of the wafer fromthe center line to the silvered strip on the left-hand side and theother being the portion from the center line to the silvered surface onthe opposite side. Polarization of those two sections would be towardone another, as indicated by the broken-construction arrows in FIGURE 4,and a silver strip along the center line Would provide one terminal. Thecorresponding sections of wafer 12' are polarized oppositely or awayfrom one another, also as indicated by broken-construction arrows inFIGURE 4, and its silvered surfaces 13, 13 would be interconnected toprovide another terminal.

A modified form of microphone is represented in FIGUREIO, differing fromthat already described in the use of two bridges Z0 and 20'. They may beindividually constructed as the bridge arrangement of FIGURE 1 and arepositioned side-by-side along the central region of maximum bending ofthe dimorph. This structure is useful to attain increased driving forcefor the dimorph and may be used for dimorphs made of certaincompositions which result in stiffer and/or heavier piezoelectricelements than those made of barium titanate.

Other forms of matching resonators, simple in structure andcomparatively inexpensive to fabricate but at the same time entirelysuited for substitution in place of resonator 20, 21 are shown inFIGURES 11, 12 and 13. In each of these figures, the view having thesubscript a is a perspective view and the view having the subscript 11is an elevation. The resonator of FIGURE 11 is a hollow cylinder open atone end and closed at its opposite end by a flexible membrane 20b whichmay be made of sheet aluminum. In the use of this resonator, the openend of the cylinder is aifixed to dimorph 11, 12 in the manner describedwith respect to the legs 21 of the bridge of FIGURE 1, cylinder wall 21bcorresponding to legs 21 and membrane 20b corresponding to bight 24}.The mounting arrangement would be essentially the same as thatheretofore described, both as to mechanical afiixation and spacerelation relative to the loop or anti-node of the dimorph.

The modification of FIGURE 12 is a T-shaped structure with a horizontalmember 20c supported by a pair of tapered leg 210. It is easilyfabricated by punching a suitably shaped section from sheet metal andsubsequently forming a T by bending. This resonator may also be directlysubstituted for the bridge of FIGURE 1 with member 200 corresponding tobight 20 and legs 21c corresponding to legs 21. It may be considered astwo cantilevers having a common mounting point and, in use as aresonator, experiences vibration in a fundamental flexural mode. If onevisualizes this structure bisected through the legs, each halfconstitutes an inverted L, or a resonator having a single cantilever.This, too, may be used in place of auxiliary resonator 20, 21 of FIGURE1.

The resonator structure of FIGURE 13 is in the form of a disc 2011 whichis center clamped to a mounting element 21d through which the disc maybe supported to a transducer. In this case, the resonator employsflexural-mode resonance of the disc 20d which corresponds to bight 20with element 21d corresponding to legs 21.

It will be understood that each of the several forms of resonatorsrepresented in FIGURES 11-13, inclusive, has an impedance at the signalfrequency which is intermediate that of the transducer and air. Further,each such resonator is constructed to have a frequency of mechanicalresonance corresponding to that of the transducer and, therefore, eachoperates in generally the same way as bridge 20, 21 of FIGURE 1 inbridging the impedance dissimilarities of the dimorph and air to attainefiicient coupling therebetween. The analogy of an electrical impedancetransformer converting between series and parallel resonance relations,described in connection with FIGURE 1, is equally applicable to theseseveral embodiments if they are mechanically connected to the dimorph ingenerally the same manner as bridge 20, 21.

All of the matching or auxiliary resonators described employ resonanceof the flexural-type which, while not a necessary limitation of thestructure, is adopted because resonators of the flexural type may beconstructed to have lower impedance values than resonators of thelongitudinal type. Additionally, flexural-mode devices are especiallydesirable where the dimensions of the transducer assembly are to be keptsmall.

As mentioned above, the concepts of this invention for achieving agreatly improved impedance match between a transducing device that has amuch higher impedance than air, or whatever the wave-signal propagatingmedium may be to which it is to be coupled, are applicable to a varietyof transducers, and the embodiment of FIGURE 14 represents schematicallythe application of a transducer of the magnetostrictive type.

The transducer of this embodiment comprises a longitudinal-moderesonator 70 in the form of an elongated rod of magnetostrictivematerial such as ferrite, nickel, or nickel/ iron alloy having strongmagnetostrictive proper-ties. It is supported mechanically in thevicinity of its longitudinal center as indicated schematically at 71,72. The resonant frequency of the rod is a function of its length and isselected to the end that the frequency of mechanical resonance isapproximately equal to the desired signal frequency. The rod issubjected to a magnetic biasing field which may be established by anelectromagnet or by a permanent magnet, a permanent magnet 73 beingrepresented for convenience of illustration. An excitation coil 74encircles the central region of the magnetostrictive rod to establishmechanical vibrations therein in response to an excitation signalsupplied from a source 75.

Since rod 70 is a longitudinal-mode resonator, the auxiliary resonator,in the form of a bridge 76 similar to bridge 2%, 21 in the embodiment ofFIGURE 1, is affixed at a free end of the resonator for the purpose ofimproving its impedance match with air. The described arrangement lendsitself particularly well to use as a sound transmitter, radiating asonic signal corresponding to the mechanical resonant frequency of therod. A signal of like frequency is applied to exciting coil 74 bygenerator 75 and the flux variations which it establishes are convertedinto mechanical stress waves which set .the rod into longitudinalvibration. This results in the radiation of a sonic signal and thebridge 76 provides eificient coupling of the transmitter to air with adesired increase in the amount of radiated sound per milliwatt ofelectrical input.

Generally, such a magnetostrictive transmitter is highly selective infrequency but the auxiliary resonator 76 permits it to operateeliiciently over a predetermined bandwidth, in addition to increasingits coupling efiiciency to air. The device is effective over a band offrequencies for essentially the same reasons described in the discussionof the frequency characteristic of FIGURE 9 since the arrangement ofFIGURE 14, instead of having a single resonant element characteristic ofthe usual magnetostrictive transmitter, has two resonant elements. Oneis the rod and the other is bridge 76. These structures have the samefrequency of mechanical resonance and, being mutually coupled, introducea saddle-type frequency response characteristic. The loading imposed onthe bridge by the radiation resistance of air fills in the saddle, so tospeak, and causes the transmitter to be effective over a band offrequencies. In other words, the structure may be likened to a properlyterminated mechanical bandpass filter.

Electromechanical transducers embodying the invention, whether employedfor purposes of reception as discussed in connection with the embodimentof FIGURE 1, or for transmission as with the arrangement of FIGURE 14,are exceedingly efiicient devices as contrasted with predecessorarrangements. They exhibit unusually high sensitivity over a desirableband of frequencies and achieve this result through the use ofstructures that are relatively simple. In particular, the auxiliarymatching resonator increases the coupling efficiency of transducerelements which are otherwise difficult to couple to air because of theirhigh impedance compared with the impedance of air. The resonator, inaddition to providing a vast improvement in coupling efficiency, alsoimposes a desired frequency response, permitting even alongitudinal-mode vibrator, which is normally quite restricted infrequency response, to be effective over a band of frequencies.Moreover, and with particular reference to a tranducer of thepiezoelectric ceramic type, the auxiliary resonator enhances theflexibility in that the resulting structure is suited for driving eithera vacuum tube or a transistor amplifier.

While particular embodiments of the invention have been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects, and, therefore, the aim in the appended claims isto cover all such changes and modifications as fall within the truespirit and scope of the invention.

I claim:

1. An arrangement for effecting transfer of energy with respect to amedium having a predetermined impedance at a given signal frequencycomprising: an electromechanical transducer having a vibratory frequencyof mechanical resonance approximately equal to said signal frequency anda mechanical impedance at said signal frequency which is high relativeto the impedance of said medium; a mechanical impedance-transformationdevice coupling said transducer to said medium and comprising aflexural-mode mechanically-resonant vibrator element coupled to saidmedium and having an impedance at said signal frequency intermediatethat of said transducer and said medium and a frequency of flexural-modemechanical resonance corresponding to that of said transducer; and meansmechanically connecting said vibrator element only to said transducer totranslate energy therebetween so that said device and said transducerconstitute a mechanical impedance transformer which is the analogue ofan electrical impedance transformer converting between parallel andseries resonant impedance relationships.

2. An arrangement for effecting transfer of energy with respect to amedium having a predetermined impedance at a given signal frequencycomprising: an electromechanical transducer vibratory in a flexural modewith a frequency of mechanical resonance approximately equal to saidsignal frequency and a mechanical impedance at said signal frequencywhich is high relative to the impedance of said medium; a mechanicalimpedance-transformation device coupling said transducer to said mediumand comprising a flexural-mode mechanically-resonant vibrator elementcoupled to said medium and having an impedance at said signal frequencyintermediate that of said transducer and said medium and a frequency offlexural-mode mechanical resonance corresponding to that of saidtransducer; and means mechanically connecting said vibrator element onlyto said transducer to translate energy therebetween so that said deviceand said transducer constitute a mechanical impedance transformer whichis the analogue of an electrical impedance transformer for convertingbetween parallel and series resonant impedance relationships.

3. An arrangement for effecting transfer of energy with respect to amedium having a predetermined impedance at a given signal frequencycomprising: an electromechanical transducer having a vibratory frequencyof mechanical resonance approximately equal to said signal frequency anda mechanical impedance at said signal frequency which is high relativeto the impedance of said medium; a mechanical impedance-transformationdevice comprising a U-shaped structure having a flexural-modemechanically-resonant 'bight portion and legs normal thereto couplingsaid transducer to said medium, said '13 bight portion having animpedance at said signal frequency intermediate that of said transducerand said medium and a frequency of fiexural-mode mechanical resonancecorresponding to that of said transducer; and means mechanicallyconnecting the free ends of said legs to said transducer to translateenergy therebetween.

4. An arrangement for effecting transfer of energy with respect to amedium having a predetermined impedance at a given signal frequencycomprising: an electromechanical transducer having a vibratory frequencyof mechanical resonance approximately equal to said signal frequencywith a mechanical impedance at said signal frequency which is highrelative to the impedance of said medium and having a line of maximumvibrational displacement; a mechanical impedance-transformation devicecoupling said transducer to said medium and comprising a flexural-modemechanically-resonant vibrator element coupled to said medium and havingan impedance at said signal frequency intermediate that of saidtransducer and said medium and a frequency of flexural-mode mechanicalresonance corresponding to that of said transducer; and meansmechanically connecting said vibrator element only to said transducerand on said line to translate energy therebetween so that said deviceand said transducer constitute a mechanical impedance transformer whichis the analogue of an electrical impedance transformer for convertingbetween parallel and series resonant impedance relationships.

5. An arrangement for effecting transfer of energy with respect to amedium having a predetermined impedance at a given Signal frequencycomprising: an electromechanical transducer device having a vibratoryfrequency of mechanical resonance approximately equal to said signalfrequency and a mechanical impedance at said signal frequency which ishigh relative to the impedance of said medium; a mechanicalimpedance-transformation device coupling said transducer to said mediumand comprising a fiexural-mode mechanically-resonant vibrator elementcoupled to said medium and having an impedance at said signal frequencyintermediate that of said transducer and said medium and a frequency offlexural-mode mechanical resonance corresponding to that of saidtransducer; means mechanically connecting said vibrator element only tosaid transducer to translate energy therebetween so that said device andtransducer have maximum vibratory response at two frequencies spacedequidistantly above and below said signal frequency, the vibratorymotion of at least one of said devices being damped to increase thevibratory response of said device and transducer intermediate said twofrequencies relative to said maximum vibratory response thereat.

6. An arrangement for effecting transfer of energy with respect to amedium having .a predetermined impedance at a given signal frequencycomprising: an electromechanical transducer having a vibratory frequencyof mechanical resonance approximately equal to said signal frequency anda mechanical impedance at said signal frequency which is high relativeto the impedance of said medium; a mechanical impedance-transformationdevice coupling said transducer to said medium and comprising aflexuralmode mechanically-resonant vibrator element coupled to saidmedium and having an impedance at said signal frequency intermediatethat of said transducer and said medium and a frequency of flexural-modemechanical resonance corresponding to that of said transducer; meansmechanically connecting said vibrator element only to said transducer totranslate energy therebetween so that said device and transducer havemaximum vibratory response at two frequencies spaced equidistantly aboveand below said signal frequency, said vibratory element beingmechanically loaded by said medium to increase the vibratory response ofsaid device and transducer interme diate said two frequencies relativeto said maximum vibratory response thereat.

7. An electromechanical transducer for effecting transfer of energy withrespect to a medium having a predetermined impedance at a given signalfrequency comprising: an electromechanical transducer having a vibratoryfrequency of mechanical resonance approximately equal to said signalfrequency with a mechanical impedance at said signal frequency which ishigh relative to the impedance of said medium and having a line ofmaximum vibratory displacement disposed in a given direction; amechanical impedance-transformation device coupling said transducer tosaid medium and comprising a flexuralmode mechanically-resonant vibratorelement coupled to said medium and having an impedance at said signalfrequency intermediate that of said transducer and said medium and afrequency of fiexural-mode mechanical resonance corresponding to that ofsaid transducer and having a line of maximum vibratory displacement; andmeans mechanically connecting said vibrator element only to saidtransducer to translate energy therebetween with said maximum vibratorydisplacement line of said element disposed in a direction normal to saidmaximum vibratory displacement line of said transducer.

8. An electromechanical transducer for effecting transfer of energy withrespect to a medium having a predetermined impedance at a given signalfrequency comprising: a piezoelectric transducer vibratory in a fiexuralmode, having a frequency of vibratory mechanical resonance approximatelyequal to said signal frequency with a mechanical impedance at saidsignal frequency which is high relative to the impedance of said mediumand having a central line of maximum vibratory displacement disposed ina given direction; a mechanical impedance transformation device couplingsaid transducer to said medium comprising a thin strip of materialformed into a U-shaped structure having a fiexural-mode mechanicallyresonant bight portion and legs normal thereto, said bight portionhaving an impedance at said signal frequency intermediate that of saidtransducer and said medium and a frequency of flexural-mode mechanicalresonance corresponding to that of said transducer and having a line ofmaximum vibratory displacement; and means mechanically connecting thefree ends of said legs to said transducer on said central line thereofwith said line of maximum vibratory displacement of said bight portiondisposed in a direction normal thereto. 7

9. An arrangement for effecting transfer of energy with respect to amedium having a predetermined impedance at a given signal frequencycomprising: an electromechanical transducer vibratory in a flexuralmode, having a frequency of flexural-mode mechanical resonanceapproximately equal to said signal frequency with a mechanical impedanceat said signal frequency which is high relative to the impedance of saidmedium and having .a central line of maximum vibratory displacementdisposed in a given direction; a mechanical impedancetransformationdevice coupling said transducer to said medium comprising a thin stripof material formed into a U-shaped structure having a fiexural-modemechanically-resonant bight portion and convergently tapered legs normalthereto, said bight portion having an impedance at said signal frequencyintermediate that of said transducer and said medium and a frequency ofmechanical resonance in a flexural mode corresponding to that of saidtransducer and having a line of maximum vibratory displacement; andmeans mechanically connecting the free ends of said legs to saidtransducer on said central line thereof with said line of maximumvibratory displacement of said bight portion disposed in a directionnormal thereto.

10. An arrangement for effecting transfer of energy with respect to amedium having a predetermined impedance at a given signal frequencycomprising: an electromechanical transducer vibratory in a flexuralmode, having a frequency of flexural-mode mechanical resonanceapproximately equal to said signal frequency with a mechanical impedanceat said signal frequency which is high relative to the impedance of saidmedium and having a central line of maximum vibratory displacementdisposed in a given direction; a mechanical impedancetransformationdevice coupling said transducer to said medium comprising a thin stripof material formed into a U-shaped structure having a flexural-modemechanically-resonant bight portion and legs normal thereto, said bightportion presenting a bending surface which is small relative to thebending surface of said transducer, having an impedance at said signalfrequency intermediate that of said transducer .and said medium and afrequency of mechanical resonance in a flexural mode corresponding tothat of said transducer and having a line of maximum vibratorydisplacement; .a tapered horn, having throat dimensions of a sizeapproximately matching said bight portion, coupling said U-shapedstructure to said medium; and means mechanically connecting the freeends of said legs to said transducer on said central line thereof withsaid line of maximum vibratory displacement of said bight portiondisposed in a direction normal thereto.

11. An arrangement for effecting transfer of energy with respect to amedium having a predetermined impedance at a given signal frequencycomprising: an electromechanical transducer vibratory in a flexuralmode, having a frequency of flexural-mode mechanical resonanceapproximately equal to said signal frequency with a mechanical impedanceat said signal frequency which is high relative to the impedance of saidmedium and having a central line of maximum vibratory displacementdisposed in a given direction; a mechanical impedancetransformationdevice coupling said transducer to said medium comprising a thin stripof material formed into a U-shaped structure having a flexural-modemechanically-resonant bight portion and legs normal thereto, said bightportion presenting a bending surface which is small relative to thebending surface of said transducer, having an impedance at said signalfrequency intermediate that of said transducer and said medium and afrequency of mechanical resonance in a flexural mode corersponding tothat of said transducer and having a line of maximum vibratorydisplacement; a tapered horn, having throat dimensions of a sizeapproximately matching said bight portion, coupling said U-shapedstructure to said medium, the mouth of said horn having a widthapproximately equal to one wavelength in said medium at said signalfrequency and having a length approximately twice said width; and meansmechanically connecting the free ends of said legs to said transducer onsaid central line thereof with said line of maximum vibratorydisplacement of said bight portion disposed in a direction normalthereto.

12. An electromechanical transducer for effecting transfer of energywith respect to a medium having a predetermined impedance at a givensignal frequency comprising: a mounting structure; a piezoelectrictransducer vibratory in a flexural mode, having a frequency offlexuralmode mechanical resonance approximately equal to said signalfrequency with a mechanical impedance at said signal frequency which ishigh relative to the impedance of said medium and having a central lineof maximum vibratory displacement disposed in a given direction; a pairof flexible, conductive electrodes secured to nodal sections of saidtransducer and mechanically aflixed to said structure for supportingsaid transducer therefrom; a mechanical impedance-transformation devicecoupling said transducer to said medium comprising a thin strip ofmaterial formed into a U-shaped structure having a flexural-modemechanically-resonant bight portion and legs normal thereto, said bightportion having an impedance at said signal frequency intermediate thatof said transducer and said medium and a frequency of flexural-modemechanical resonance corresponding to that of said trans ducer andhaving a line of maximum vibratory displacement; and means mechanicallyconnecting the free ends of said legs to said transducer on said centralline thereof 26 with said line of maximum vibratory displacement of saidbight portion disposed in a direction normal thereto.

13. An electromechanical transducer for effecting transfer of energywith respect to a medium having a predetermined impedance at a givensignal frequency comprising: an electromechanical transducer having avibratory frequency of mechanical resonance approximately eqaul to saidsignal frequency with a mechanical impedance at said signal frequencywhich is high relative to the impedance of said medium and having a lineof maximum vibratory displacement disposed in a given direction; aplurality of mechanical impedance-transformation devices coupling saidtransducer to said medium, each comprising a fiexural-modemechanically-resonant vibrator element coupled to said medium and havingan impedance at said signal frequency intermediate that of saidtransducer and said medium and a frequency of flexural-mode mechanicalresonance corresponding to that of said transducer; and meansmechanically connecting each of said vibrator elements only to saidtransducer with said elements disposed side-by-side along said line totranslate energy between said elements and said transducer and so thateach ical impedance transformer which is the analogue of an electricalimpedance transformer for converting between series and parallelimpedance relationships in a resonant circuit.

14. An arrangement for effecting transfer of energy with respect to amedium having a predetermined impedance at a given signal frequencycomprising: an electromechanical transducer having a vibratory frequencyof mechanical resonance approximately equal to said signal frequencywith a mechanical impedance at said signal frequency which is highrelative to the impedance of said medium; a mechanicalimpedance-transformation device coupling said transducer to said mediumcomprising a cylinder closed at one end by a flexible membrane vibratoryin a flexural mode, said membrane having an impedance at said signalfrequency intermediate that of said transducer and said medium and afrequency of flexural-mode mechanical resonance corresponding to that ofsaid transducer; and means mechanically connecting the other end of saidcylinder to said transducer.

15. An arrangement for effecting transfer of energy with respect to amedium having a predetermined impedance at a given signal frequencycomprising: an electromechanical transducer having a vibratory frequencyof mechanical resonance approximately equal to said signal frequencywith a mechanical impedance at said signal frequency which is highrelative to the impedance of said medium; a mechanicalimpedance-transformation device coupling said transducer to said mediumcomprising a T-shaped structure defining two balanced cantileversvibratory in a fundamental flexural mode, said cantilevers having animpedance at said signal frequency intermediate that of said transducerand said medium and a frequency of flexural-mode mechanical resonancecorresponding to that of said transducer; and means mechanicallyconnecting the said T-shaped structure to said transducer.

16. An arrangement for effecting transfer of energy with respect to amedium having a predetermined impedance at a given signal frequencycomprising: an electromechanical transducer having a vibratory frequencyof mechanical reasonance approximately equal to said signal frequencywith a mechanical impedance at said signal frequency which is highrelative to the impedance of said medium; a mechanicalimpedance-transformation device coupling said transducer to said mediumcomprising a disc center-clamped to one end of a mounting element andvibratory in a flexural mode, said disc having an impedance at saidsignal frequency intermediate that of said transducer and said mediumand a frequency of flexuralm'ode mechanical resonance corresponding tosaid transducer; and means mechanically connecting the other end of saidmounting element to said transducer.

17. An arrangement for effecting transfer of energy with respect to amedium having a predetermined impedance at a given signal frequencycomprising: an elongated longitudinal-mode resonator having a vibratoryfrequency of longitudinal-mode mechanical resonance approximately equalto said signal frequency with a mechanical impedance at said signalfrequency which is high relative to the impedance of said medium; amechanical impedancetransformation device coupling said resonator tosaid medium and comprising a flexural-mode mechanically-resonantvibrator element coupled to said medium and having an impedance at saidsignal frequency intermediate that of said resonator and said medium anda frequency of ficxural mode mechanical resonance corresponding to thatof said resonator; and means mechanically connecting said element onlyto one end surface of said resonator.

18. An arrangement for effecting transfer of energy with respect to amedium having a predetermined impedance at a given signal frequencycomprising: a magnetostrictive transducer including an elongatedlongitudinalmode resonator having a longitudinal-mode vibratoryfrequency of mechanical resonance approximately equal to said signalfrequency with a mechanical impedance at said signal frequency which ishigh relative to the impedance of said medium; a mechanicalimpedance-transformation device coupling said transducer to said mediumand comprising a fiexural-mode mechanically-resonant vibrator elementcoupled to said medium and having an impedance at said signal frequencyintermediate that of said transducer and said medium and a frequency offiexural-mode mechanical resonance corresponding to that of saidtransducer; and means mechanically connecting said vibrator element onlyto said transducer to translate energy therebetween so that said deviceand said transducer constitute a mechanical impedance transformer whichis the analogue of an electrical impedance transformer convertingbetween parallel and series resonant impedance relationships.

References Cited in the file of this patent UNITED STATES PATENTS2,168,809 Semple Aug. 8, 1939 2,242,755 Pope r May 20, 1941 2,487,962Arndt Nov. 15, 1949 2,573,168 Mason et a1 Oct. 30, 1951 2,592,703 JaffeApr. '15, 1952'

